1. Background of the Invention
In an oven controlled crystal oscillator, in particular because an operation temperature of the crystal resonator can be kept constant, a highly stable oscillation frequency of an order of, for example, 0.1 ppm or less or 1 ppb can be obtained without causing frequency change dependent on a frequency-temperature characteristics. These oven controlled crystal oscillators are used particularly as fixed stations in communication equipment.
2. Prior Art
FIG. 4 is a diagram for explaining an oven controlled crystal oscillator in a conventional example described in Patent Document 1, wherein FIG. 4A is a sectional view of the oven controlled crystal oscillator, and FIG. 4B is a plan view of a circuit board as seen from the direction A in FIG. 4A. FIG. 4A is a sectional view on arrow IV-IV in FIG. 4B.
As shown in FIG. 4, an oven controlled crystal oscillator 1 is configured by enclosing and sealing a circuit board 2 having an oscillation circuit and a temperature control circuit formed thereon, in a container comprising a metal base 3 and a metal cover 4. On the circuit board 2 is mounted a heat-conducting plate 6 on which is mounted a crystal resonator 5, an oscillation element 7 constituting an oscillation circuit together with the crystal resonator 5, a thermistor 8 serving as a temperature sensor that detects the temperature of the crystal resonator 5, a heating resistance 9 that heats the crystal resonator 5 depending on the temperature detected by the thermistor 8, and a temperature control element 10 comprising a power transistor 10a and the like constituting the temperature control circuit. Hereunder is a details description.
The oven controlled crystal oscillator 1 includes the circuit board 2 made from a glass epoxy material provided with a wiring pattern (not shown in the drawing). As shown in FIG. 4B, the rectangular heat-conducting plate 6 made of aluminum is mounted on one surface of the circuit board 2. The heat-conducting plate 6 is screw attached to the circuit board 2 by threading screws 11 at the four corners thereof. On the four side faces of the heat-conducting plate 6 there are formed open areas 12a to 12d opened on the four side faces and passing through in a thickness direction of the heat-conducting plate 6. Moreover a central through hole 13 is formed in the center of the heat-conducting plate 6.
The crystal resonator 5 with lead wires (oscillator lead wires 14) extending from an external bottom face (upward in FIG. 4A) is mounted on the surface of the heat-conducting plate 6 opposite to the surface facing the circuit board 2. The crystal resonator 5 has an SC-cut crystal piece (not shown in the drawing) housed thereinside, and the crystal piece is electrically connected to the oscillator lead wires 14. The oscillator lead wires 14 are inserted into through holes formed on the heat-conducting plate 6 to be joined to the circuit board 2, and are electrically connected to the oscillation element 7 constituting the oscillation circuit mounted on the circuit board 2. Here the oscillation element 7 includes, other than the oscillation circuit (for example, a Colpitts-type essential element), a capacitor, an inductor, a transistor as a buffer amplifying circuit, and the like according to application.
Moreover, the temperature control circuit for maintaining temperature of the crystal resonator 5 constant, is formed on the circuit board 2. The temperature control circuit includes the thermistor 8 as a temperature sensor that detects the temperature of the crystal resonator 5, the heating resistance 9 that heats the crystal resonator 5 depending on the temperature detected by the thermistor 8, and the temperature control element 10 comprising the power transistor 10a and the like.
The thermistor 8 is arranged inside the central through hole 13 of the heat-conducting plate 6. Moreover a chip resistance is used as the heating resistance 9, and is arranged inside the open areas 12b and 12d. Furthermore the power transistor 10a is arranged inside the open areas 12a and 12c. An amount of heat discharged by the power transistor 10a together with the heating resistance 9 is used for heating the crystal resonator 5.
Slits 15 are provided around the four corners of the heat-conducting plate 6 provided on the circuit board 2, thereby preventing an amount of heat generation of the heating resistance 9 and the power transistor 10a from escaping to the outside from the heat-conducting plate 6, so as to efficiently heat the crystal resonator 5. Such a circuit board 2 is enclosed and sealed inside the container comprising of the metal base 3 and the metal cover 4.
The metal base 3 is a flat plate in a plan view, and lead wires (oscillator lead wires 16a to 16d) penetrating the metal base 3 are provided at the four corners thereof serving as connecting terminals of the oven controlled crystal oscillator 1 (airtight terminals serving as the lead wires). The oscillator lead wire 16a which is the earth, is connected to the metal base 3 using silver solder 17, and the oscillator lead wires 16b to 16d other than the earth, which are supply terminals, output terminals, or the like, are insulated and connected inside through holes 18a of the metal base 3 by using glass 18.
The circuit board 2 is held by the oscillator lead wires 16a to 16d, and is arranged above the metal base 3. The oscillator lead wires 16a to 16d are electrically connected to the oscillation element 7 and the temperature control device 10 on the circuit board 2. Moreover the metal cover 4 is connected to the metal base 3 by resistance-welding, and the circuit board 2 is enclosed and sealed in the metal cover 4.
In such a conventional example, when the ambient temperature of the oven controlled crystal oscillator 1 changes to change the temperature of the crystal resonator 5, the thermistor 8 detects the temperature change of the crystal resonator 5, and the amount of heat generation of the heating resistance 9 is adjusted by the temperature control circuit. As a result the temperature of the crystal resonator 5 is kept constant (for example, about 85° C.). Accordingly, a change in the oscillation frequency can be suppressed.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2005-333315